Final answer:
Chromosomes are responsible for about half of all spontaneous abortions in humans.
Step-by-step explanation:
In humans, about one-half of all spontaneous abortions are caused by abnormalities in chromosomes. Chromosomal abnormalities can result in the loss of the pregnancy during the early stages. These abnormalities can occur due to errors in the formation or structure of the chromosomes, leading to developmental problems in the fetus.